SOME SPECIAL PKOCESSES 281 



Blue Screen. 



Gelatin solution in distilled water, 1 in 15 . 40 c.c. 

 Patent blue solution, 1 in 1000 ... 2 c.c. 



7 c.c. of this solution are used for each square decimetre of glass. 



The screens are coated with the gelatin solutions, and dried 

 in the manner already described, and are then bound together. 



For incandescent gaslight the same formulae may be used, 

 except that the blue screen is 1 required a little paler, 5 c.c. being 

 used instead of 7 c.c. per decimetre of surface. It must be 

 borne in mind that with either of these sources of light some 

 alteration in their spectroscopic value takes place as they 

 are used. It is better not to use either a Nernst-lamp filament 

 or a Welsbach gas-mantle for a very lengthened period, as their 

 light-efficiency tends to decrease, and the amount of red light 

 they emit increases. When working with the electric arc it 

 must be admitted that difficulties may be encountered. There 

 is the variability in the carbons, and the inevitable variation 

 in the efficiency of the arc itself which may affect the results, 

 although in ordinary photographic work this would not become 

 apparent. A screen for use with the electric arc may be made 

 as follows : 



Gelatin solution in distilled water, 1 in 15 . 40 c.c. 



Tartrazine solution, 1 in 500 ... 4 c.c. 



Saffranine solution, 1 in 7000 ... 1 c.c. 



Aesculin, dissolved in 35 c.c. of water, with 



the addition of 3 drops of ammonia . 0*4 grm. 

 This solution is suitable for an arc taking from 15 to 25 amperes, 

 and 8 c.c. are used for each square decimetre of surface. For 

 arcs taking less current, the strength of the solution may 

 conveniently be reduced somewhat so that a screen of less 

 absorptive power is obtained. 



A screen suitable for lime-light is as follows : 

 Stock solutions. 



A. Quinoline yellow . . . . 9 '5 grm. 

 Distilled water . . . . . 100 c.c. 



B. Brilliant acid-green 6 B . . . O'l grm. 

 Distilled water 100 c.c. 



In 5 c.c. of distilled water there is dissolved 0'05 gramme 



